1. Introduction
This invention relates to an electrolytic plating solution used for uniformly depositing a metal coating on the walls of through-holes in multilayer printed circuit boards. In particular, the invention relates to an additive to the electrolytic plating solution which functions to increase the throwing power of the solution without sacrificing leveling or the ability of the deposit to resist thermal stress.
2. Description of the Prior Art
Methods for electroplating articles with metal coatings are well known in the art. Such methods involve passing a current between two electrodes in a plating solution where one of the electrodes is the article to be plated. An acid copper plating solution would comprise dissolved copper (usually copper sulfate), an acid electrolyte such as sulfuric acid in an amount sufficient to impart conductivity to the bath, and proprietary additives to improve the efficiency of the plating reaction and the quality of the metal deposit. Such additives include surfactants, brighteners, levelers, suppressants, etc.
Electrolytic copper plating solutions are used for many industrial applications. For example, they are used in the automotive industry as base layers for subsequently applied decorative and corrosion protective coatings. They are also used in the electronics industry, particularly for the fabrication of printed circuit boards. For circuit fabrication, copper is electroplated over selected portions of the surface of a printed circuit board and onto the walls of through-holes passing between the surfaces of the circuit board base material. The walls of a through-hole are metallized to provide conductivity between circuit layers on each surface of the board.
Early efforts to make circuit boards used electrolytic copper plating solutions developed for decorative plating. However, as printed circuit boards became more complex and as industry standards became more rigorous, solutions used for decorative plating were found to be inadequate for circuit board fabrication. A serious problem encountered using electrolytic copper plating solutions involved coatings of uneven thickness on the walls of the through-hole with the deposits thicker at the top and bottom of the holes and thinner at the center, a condition known in the art as "dog boning". The thin deposit at the center of the through-hole may lead to circuit defects and board rejection.
Dog boning is caused by a voltage drop between the top surface of the hole and the center of the hole. This potential drop is a function of current density, a ratio of the length of the hole to its diameter (aspect ratio) and board thickness. As the aspect ratio and the thickness of the board increase, dog boning becomes more severe due to a voltage drop between the surface of the board and the center of the through-hole. This voltage drop is caused by a combination of factors including solution resistance; a difference in surface to hole overpotential due to mass transfer--i.e., a difference in the flow of solution through the hole compared to the movement of the solution over the surface of the board; and a charge transfer difference as a consequence of the concentration of additives in the hole as compared to the surface.
The circuit board industry continuously seeks greater circuit densification. To increase density, the industry has resorted to multilayer circuits with through-holes or interconnections passing through multiple layers. Multilayer circuit fabrication results in an overall increase in the thickness of the board and a concomitant increase in the length of an interconnection passing through the board. This means that increased circuit densification results in increased aspect ratios and hole length and an increase in the severity of the dog boning problem. For high density boards, aspect ratios typically exceed ten to one.
The prior art, exemplified by Mayer and Barbien, "Characteristics of Acid Copper Sulfate Deposits for Printed Wiring Board Applications", Plating and Surface Finishing, pp. 46 to 49, March, 1981; Malak, "Acid Copper Plating of Printed Circuits", Products Finishing, pp. 38 to 44, March, 1981; and Amadi, "Plating High Aspect Ratio Multilayer Boards", PC FAB, pp. 85 to 94, October, 1987, all incorporated herein by reference, suggest that increasing the acid to metal ion ratio of an electrolytic plating solution improves plating solution throwing power and deposit distribution. The prior art teaches that the ratio may be altered by increasing acid concentration while holding metal ion concentration constant; or by decreasing metal ion concentration while holding acid concentration constant. The use of such methods to improve throwing power may lead to anode polarization with cessation of the plating reaction or decreased metal concentration which exacerbates the dog boning problem.
For a given set of chemical and physical conditions, the surface-to-hole ratio increases with increasing aspect ratio (board thickness divided by the hole diameter). For an aspect ratio of less than 3.0 and with a good plating additive, the effect is minimal and plating distribution in the hole is fairly uniform with the surface-to-hole ratio approaching 1.0. With increasing aspect ratio, both physical and chemical parameters must be optimized to obtain uniform plating in the holes. Copper plating additives may be used to optimize the plating. In Amadi, it is suggested that the additives may be multicomponent organic materials and that only the brightener and the carrier have been found to affect plating distribution in high aspect ratio multilayer boards. Amadi further suggests that increasing the carrier level improves the plating distribution in the holes but that there is also a critical concentration under which the brightener will not perform.
U.S. Pat. No. 4,683,036 describes a method for electroplating non-metallic surfaces of plating holes in metal clad laminates. The process involves use of a bath component which will cause preferential plating on two different metals. The present invention differs from the above patent in that preferential plating is not involved; and in the patent, although the component may aid in the initial coverage, once a thin layer of copper is plated, the system would no longer have sufficient resistance to continue the preferential plating.
Plating of thick, high aspect ratio through-hole boards is controlled by two fundamental areas of the process, fluid dynamics and electrochemistry. Fluid dynamics deals with the necessary to replenish metal ions in the hole. During
plating of high aspect ratio holes, there must be sufficient exchange of metal ion depleted solution within the hole with fresh solution. Otherwise, the plating reaction in the hole will become strongly polarized causing the deposition rate there to slow relative to that on the surface. It has been shown that very small pressures are required to provide the necessary through hole agitation to ensure a nearly constant copper concentration in the hole and it is well known in the art that these pressures are easily attained using modest cathode movement.
The effect of the copper metal:sulfuric acid ratio has previously been discussed in the copending patent (U.S. Pat. No. 4,897,165). The patent discloses the use of a high molecular weight surfactant in an optimized acid-copper bath, which improves throwing power within the through-hole opening.
The role of electrochemistry of the plating bath provides the greatest area for improvements in the process. Throwing power is defined as the ratio of the hole center copper deposit thickness to its thickness at the surface. For plating high aspect ratio holes in printed circuit boards, this ratio should be greater than about 0.5:1. Optimum throwing power is achieved when the plating current density at the center of the hole is the same as that flowing on the board surface.
It has been discovered, through the use of electrochemical studies, that the plating current density is dependent upon the overpotential, or energy available for electrodepositing copper. Measurement of the overpotential for a particular bath provides a means of controlling the throwing power of the bath and bath composition as the bath ages. Plating additives have a marked effect on charge transfer overpotential and can be used to favorably affect the throwing power.
Conventional prior art baths have not been able to plate high aspect ratio holes in thick printed circuit boards where the requirements are for low operating current densities, exceptional throwing power, bright deposits and excellent mechanical properties of the deposit. The disclosed invention addresses all of these requirements.